1. Field of the Invention
The present invention relates, for example, to a flat panel display. More particularly it relates, for example, to a flat panel display that can improve light coupling efficiency of a light emitting device and help to prevent loss of image sharpness.
2. Description of the Related Art
Light coupling efficiency of a flat panel display using a light emitting device is determined by the refractive index of each layer from the light emitting layer to the outside of the flat panel display. Light coupling efficiency is lower if light exits in an undesired direction when light is transmitted from a transparent substrate having a higher refractive index to air having a lower refractive index. Also, light coupling efficiency is lower when light is incident on a boundary surface between the substrate and air at an angle greater than the critical angle, the light is totally reflected (not transmitted outwardly).
In a conventional flat panel display using a light emitting device as shown in FIG. 1, the light coupling efficiency when light emitted from the light emitting device 1 is transmitted to air in a transparent substrate 2 is based on the formula ½(Nout/Nin)2, in which N is a refractive index.
When light coupling efficiency is calculated based on the above formula for a case in which a transparent glass substrate is used light coupling efficiency is about 21.64%. This is because the refractive indexes of the transparent glass substrate and the air are about 1.52 and 1.00, respectively. Thus, 70% or more of the light incident on the transparent substrate 2 is lost in the transparent substrate 2. Also, since light emitted from the light emitting device travels in an undesired direction, there have been various efforts to solve this problem.
One of the efforts was to increase the supply voltage, and thereby improve brightness. However, this requires increasing the capacity of, or shortening the duration of use of the battery (if a battery is used to power the device). If battery capacity is increased, this can increase the weight of the device as a whole. Therefore, the some references have attempted to improve brightness while lowering the supply voltage.
Japanese Patent Publication No. hei 9-73983 discloses an electroluminescent (EL) lighting device in which an acryl resin based prism lens sheet (on which a plurality of prisms having lengthwise axes parallel to one another are formed) is installed on a light emitting surface of an EL sheet. Total reflection is reduced by allowing light incident on an interface between a transparent substrate and air at an angle greater than the critical angle to have an incident angle less than the critical angle at each side of the prism lens. Brightness in a predetermined direction is increased by allowing light to exit in the predetermined direction. However, some light is lost due to total reflection in the prism lens. Additionally, the sharpness of the image is degraded by overlapping of the image.
To solve the problem that total reflection exists in the prism lens, Korean Patent Publication No. 2003-0096509 discloses a prism sheet. The prism sheet includes a first surface on which light emitted from a light source is incident, a second surface through which light incident from the first surface exits. The second surface including a plurality of polygonal protruding members having lengthwise directional axes parallel to one another. Each of the protruding members has at least one first side forming an angle of about 65°-75° with a line perpendicular to the first surface and at least one second side adjacent to the first side and at an angle of about 40°-60° with the line perpendicular to the first surface. Such a configuration can help to reduce total reflection in the prism.
However, the above invention is not used for the light emitting device but for the backlight. When the invention is used for the flat panel display using a light emitting device, the image sharpness deteriorates because of the overlapping of the image.